Microlithography is used for producing microstructured components, such as for example integrated circuits. The microlithographic process is carried out with a lithography apparatus, which has an illumination system and a projection system. The image of a mask (reticle) illuminated via the illumination system is thereby projected via the projection system onto a substrate (for example a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection system, in order to transfer the mask structure to the light-sensitive coating of the substrate.
Driven by the desire for ever smaller structures in the production of integrated circuits, currently under development are EUV lithography apparatuses that use light with a wavelength in the range from 0.1 nm to 30 nm, in particular 13.5 nm. In the case of such EUV lithography apparatuses, because of the high absorption of light of this wavelength by most materials, reflective optics, that is to say mirrors, are used instead of refractive optics, that is to say lenses.
The mirrors may for example be fastened to a supporting frame (force frame) and be designed as at least partially manipulable, in order to allow a movement of a respective mirror in up to six degrees of freedom, and consequently a highly accurate positioning of the mirrors in relation to one another, in particular in the picometer range. This allows changes in the optical properties that occur for instance during the operation of the lithography apparatus, for example as a result of thermal influences, to be compensated.
For mounting the mirrors on the supporting frame, usually weight compensating devices on the basis of permanent magnets (magnetic gravity compensators) are used, as described for example in DE 10 2011 088 735 A1. Such a weight compensating device includes for example a housing which is coupled to the supporting frame and a holding element which is movable with respect to the housing and is coupled to the mirror. Fastened to the holding element are for example two ring magnets (permanent magnets), which together with a ring magnet (permanent magnet) arranged on the housing generate a compensating force in the vertical direction. The compensating force acts counter to the weight of the mirror, and corresponds substantially to it in terms of its absolute value.
By contrast, the movement of a respective mirror—in particular also in the vertical direction—is actively controlled by way of so-called Lorentz actuators. Such a Lorentz actuator respectively includes an energizable coil and, at a distance from it, a permanent magnet. These together generate an adjustable magnetic force for controlling the movement of the respective mirror. One or more Lorentz actuators may be integrated in the weight compensating device, as described for example in DE 10 2011 004 607 A1. In this case, the coil of the Lorentz actuator is arranged in the housing and acts on the two ring magnets arranged on the holding element.
It is problematic however that the compensating force generated by the weight compensating device can change over time. For example, the magnetic forces of the magnets used weaken as a result of aging. The then insufficient (or excessive) weight compensation is balanced out via the Lorentz actuators, which leads to a constant flow of current in the coils of the actuators. The constant flow of current in turn creates a heat source, with potentially adverse consequences for the positioning of the corresponding mirror.
An approach to solving this problem is described in DE 10 2011 088 735 A1. This provides a vertically displaceable ring of magnetically soft material arranged around the weight compensating device. Depending on the position of the ring, it influences the magnetic field of the weight compensating device correspondingly, in order thereby to adjust the compensating force. A disadvantage of this solution is that there is an additional mechanical component, which has to be moved correspondingly.